The fermentation process is where the microbial reaction of bacteria and yeasts break down the sugars in the mucilage to produce acids.
It’s these acids that will be responsible for adding depth and complexity to the coffee. There are three main ways of processing the harvested cherries through the fermentation stage. Each process has its own logistical pros and cons and the process can have a significant effect on the taste of the final product.
Low Fermentation (Wet Processing)
Coffee beans in the ferment and wash method.
[nimon_t] / Depositphotos.com
This is the more modern, quicker process but it uses a lot of water. It has become the most common way of fermenting coffee.
The pulped beans are sorted by size and then thrown into fermentation tanks.
After 12 to 48 hours of fermentation in the tank, the naturally occurring enzymes dissolve the layer of mucilage surrounding the beans.
The beans are then washed thoroughly in fresh water to stop the fermentation process and to remove the last of the pulp.
This leaves the beans covered in just a thin sheath, or parchment, called the endocarp.
This process allows the farmer to carefully control how much fermentation takes place and results in a more consistent coffee with clean and complex flavors.
Medium Fermentation (Semi-washed)
For this method the cherries have their skins removed during the pulping process but instead of completely removing the mucilage, as in the wet process, the sticky flesh layer is left around the beans.
This allows for some measure of fermentation to continue throughout the drying process. This is also known as Honey or Pulped Natural coffee.
There’s actually no “washing” that takes place, semi or otherwise, so you’ll have to ask someone in a lab coat why they call it “semi-washed” because I don’t know.
The end result, though, is a coffee with a fruitier taste and more body than you get from the wet process.
A pulper machine is used to mechanically remove the skin of the coffee cherry.
High Fermentation (Dry process)
This is the oldest method and is still used in many coffee producing countries where water is scarce.
The ripe, freshly picked cherries do not go through the pulping process but are spread out, skin and all, on a large even surface to ferment while drying in the sun.
Because the skins are left on and the cherries aren’t all lying in the same tank each one ferments a little differently to the other.
This makes it a challenge to control the fermentation and get consistency from the coffee. When it’s done right, though, it delivers the most complex and intense flavors with great body.